This invention relates to pumps for continuous high pressure pumping of noxious liquids and liquid gases, and more particularly, to a hermetically sealed magnetic drive pump having condition responsive control.
The continuous pumping of gases in liquid phase, organic fluids, noxious liquids and other volatile liquids, requires high pressure pumping processes. Examples of such volatile liquids include propane, ammonia, and ethanol. Volatile liquids must be confined under pressure or they will vaporize resulting in vapor lock of the pump. Therefore, a high discharge pressure must be maintained to provide a constantly liquid filled pump in operation. To aid in maintaining a high discharge pressure and to prevent vaporization of the processed liquid, pumps are designed with the low NPSM and high head characteristics of the regenerative turbine pump combination.
A further consideration is that protection must be provided against environmental hazards, particularly when potentially hazardous fluids, such as propane and ammonia, are being pumped. It is becoming common practice to provide secondary containment for pumps designed to pump volatile liquids to guard against the occurrence of a hazardous condition. One way to accomplish secondary containment in such pumps is to employ a hermetically sealed magnetic drive for the pump and to enclose the sealed magnetic drive within a housing. Such pumps are commonly referred to as "canned" magnetic drive pumps. Because the hermetically sealed magnetic drive of the pump is sealed within a containment vessel, the heat generated tends to build up within the containment vessel. The heat is generated by moving parts, such as, bearing which are contained within the containment vessel. In addition, the relative motion of the driving magnet and the driven magnet of the magnetic drive and the containment vessel produces eddy currents in the containment vessel, resulting in heat build-up. The heat must be dissipated to prevent damage to the magnetic drive.
Various cooling arrangements have been proposed for canned magnetic drive pumps. Typically, heat build-up is minimized by directing a portion of the liquid at the high discharge pressure through the containment vessel as a coolant or heat transfer medium to remove heat from the interior of the containment vessel. Conventionally, the liquid is returned to the suction inlet of the pump to be recirculated through the pump. However, because the liquid is heated as it is circulated through the containment vessel, the possibility exists that the high pressure liquid being returned to the section inlet for recirculation will "flash" into vapor at the low pressure suction inlet. Moreover, although circulation of a portion of the processed fluid through the containment vessel reduces heating within the containment vessel, under some conditions, the coolant flow may not provide adequate cooling, or coolant flow may be interrupted for some reason, resulting in overheating and possible damage to the pump assembly and in particular to the magnetic drive for the pumping stage.
A further shortcoming of this heat transfer method is that the liquid that is circulated through the containment vessel picks up contaminants, including magnetic particles and metal, which are reintroduced into the flow stream at the suction inlet. Such contaminants will be circulated through the pumping stage and may cause damage to the pump stage and the magnetic drive bearings.